1. Field of the Invention
This invention is directed to a semiconductor device and a manufacturing method of the semiconductor device. More particularly to a semiconductor device having a gate insulating film structure including an insulating film which contains metal, silicon and oxygen, for example, a metal silicate film and the manufacturing method thereof.
2. Description of the Related Art
It is desirable to make a thin gate insulating film for the miniaturization of the Metal Insulator Semiconductor Field Effect Transistor (“MISFET”). If a thin silicon oxide film or silicon oxide-nitride film is used as the gate insulating film, a leak current, that is, a direct tunnel current is increased due to a reduction in the film thickness. As a result, the minimum thickness is approximately 2 nm.
Therefore, it is proposed to suppress a leak current by using a metal oxide film or metal silicon-oxide film (hereinafter referred to as a metal silicate film) having a dielectric constant larger than that of the silicon oxide film. That is, when a metal silicate film or a metal oxide film is used as a gate insulating film, the film thickness for attaining an electrical characteristic equivalent to that of a silicon oxide film (“effective film thickness”) can be reduced.
However, when the metal oxide film is used as the gate insulating film, it is difficult to obtain a good interface which is free from distortion in a portion facing a silicon substrate. Additionally, when the metal silicate film is used as the gate insulating film, a mediocre interface is attained, with a smaller dielectric constant than that of the metal oxide film.
It was postured to form a metal silicate film at the interface between a silicon substrate and a metal oxide film by heat treatment after formation of the metal oxide film on the silicon substrate so as to form a laminated structure of the two films on the silicon substrate. Unfortunately, with time, the constituent metals of the two films become the same. Therefore, it is difficult to attain the best combination of the metal oxide film and metal silicate film by using different kinds of the constituent metals of the two films. Additionally, since metal oxide film is made of a crystalline metal oxide material, the effective film thickness locally varies due to its dependency on the dielectric constant of the crystal plane orientation.
The metal oxide film and metal silicate film are formed by a Chemical vapor Deposition (“CVD”) method, but a metal oxide film formed by the CVD method has a composition slightly different from a stoichiometric composition and tends to become a coarse film with a low density.
When two MISFETs with different gate insulating film thickness are formed on the same semiconductor substrate, the process of forming a metal oxide film on the entire surface of the substrate, removing part of the metal oxide film on the gate region of one of the MISFETs whose gate insulating film is made thin and forming a metal oxide film on the entire surface of the resultant structure are required. Unfortunately, this process is complicated and inefficient.
Thus, using a metal oxide film or metal silicate film as an insulating film a dielectric constant larger than that of a silicon oxide film is proposed. However, there are many problems in maintaining the characteristics of the gate insulating film while attaining the reduction in the physical and/or the actual film thickness. Specifically, problems with respect to the characteristic of the resulting elements and the productivity of manufacturing the elements.